U.S. patent number 11,388,539 [Application Number 17/128,529] was granted by the patent office on 2022-07-12 for method and device for audio signal processing for binaural virtualization.
This patent grant is currently assigned to Sennheiser electronic GmbH & Co. KG. The grantee listed for this patent is Sennheiser electronic GmbH & Co. KG. Invention is credited to Renato Pellegrini.
United States Patent |
11,388,539 |
Pellegrini |
July 12, 2022 |
Method and device for audio signal processing for binaural
virtualization
Abstract
Binaurally reproduced audio signals are often perceived as
unnatural. For example, speech intelligibility may be reduced. For
improving the spatial reproduction of audio signals, the invention
enables binaurally virtualizing a single-channel audio signal only
partially by filtering. A degree of binaural virtualization for the
audio signal based on one or more processing parameters (P.sub.C,
P.sub.FC, P.sub.TC) may be freely chosen. A control allows a smooth
transition between a completely binaural virtualization based on
HRTF and a non-binaural virtualization corresponding to panning. A
first range (B.sub.1) starts with a completely binaural
virtualization and the HRTFs that are commonly used for this. In
this range, the HRTFs are modified by scaling and by approaching
them to the gain factors of the panning while decreasing a degree
of binaural virtualization. In a subsequent second range (B.sub.2)
that leads to a completely panning-like virtualization, the
resulting phase is reduced, or adjusted to the panning phase of
0.degree.. By selecting one or more processing parameters,
different audio signals may be binaurally virtualized to different
degrees before being superposed to each other.
Inventors: |
Pellegrini; Renato
(Niederhasli, CH) |
Applicant: |
Name |
City |
State |
Country |
Type |
Sennheiser electronic GmbH & Co. KG |
Wedemark |
N/A |
DE |
|
|
Assignee: |
Sennheiser electronic GmbH &
Co. KG (Wedemark, DE)
|
Family
ID: |
1000006424556 |
Appl.
No.: |
17/128,529 |
Filed: |
December 21, 2020 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20210195361 A1 |
Jun 24, 2021 |
|
Foreign Application Priority Data
|
|
|
|
|
Dec 23, 2019 [DE] |
|
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102019135690.3 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04S
7/303 (20130101); H04S 1/007 (20130101); H04S
2420/01 (20130101); H04S 2400/11 (20130101); H04S
2400/13 (20130101) |
Current International
Class: |
H04S
7/00 (20060101); H04S 1/00 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Zhu; Qin
Attorney, Agent or Firm: Haug Partners LLP
Claims
The invention claimed is:
1. A method for processing an input audio signal, the method
comprising: assigning a direction and at least one processing
parameter for a degree of binaural virtualization to the input
audio signal; determining a first head-related transfer function
for a left output signal for a left-side ear of a listener and a
second head-related transfer function for a right output signal for
a right-side ear of the listener, wherein the first and second
head-related transfer functions correspond to the direction
assigned to the input audio signal; determining a first gain factor
for the left side and a second gain factor for the right side,
wherein the first and second gain factors correspond to an
amplitude panning for the direction assigned to the input audio
signal; modifying an amplitude response of the first head-related
transfer function according to the processing parameter to bring
the amplitude response closer to the first gain factor, wherein a
first modified head-related transfer function is obtained;
modifying an amplitude response of the second head-related transfer
function according to the processing parameter to bring the
amplitude response closer to the second gain factor, wherein a
second modified head-related transfer function is obtained; wherein
at least in a first frequency range the amplitude responses for a
lower degree of binaural virtualization are brought closer to the
respective gain factor than for a higher degree of binaural
virtualization; calculating a first filter according to the first
modified head-related transfer function and a second filter
according to the second modified head-related transfer function;
filtering the input audio signal with the first filter and the
second filter, wherein a filtered audio signal each for the left
ear and the right ear of the listener is obtained that is partially
binaurally virtualized according to said assigned degree.
2. The method according to claim 1, wherein the input audio signal
is one of a mono signal, a channel of a channel-based audio signal
and an audio object of an object-based audio signal.
3. The method according to claim 1, wherein said modifying the
amplitude response of the first head-related transfer function and
said modifying the amplitude response of the second head-related
transfer function comprises: transforming the first and second
head-related transfer functions into the frequency domain by means
of a Fourier transformation, wherein a transformed first
head-related transfer function and a transformed second
head-related transfer function are obtained; calculating a first
amplitude response for the first head related transfer function and
a second amplitude response for the second head related transfer
function; interpolating according to the processing parameter
between the amplitude frequency response of the transformed first
head-related transfer function and the determined first gain
factor, wherein a transformed first modified head-related transfer
function is obtained; interpolating according to the processing
parameter between the amplitude frequency response of the
transformed second head-related transfer function and the
determined second gain factor, wherein a transformed second
modified head-related transfer function is obtained; and
re-transforming the transformed first and second modified
head-related transfer functions into the time domain, wherein the
first and second modified head-related transfer functions are
obtained.
4. The method according to claim 3, further comprising: determining
a first group delay of the first head-related transfer function and
a second group delay of the second head-related transfer function;
subtracting the determined first group delay from the phase
response of the transformed first head-related transfer function,
whereby a normalized first phase response results; unwrapping the
normalized first phase response, wherein phase jumps in the
normalized first phase response are eliminated by adding or
subtracting a value of 360.degree. or multiples thereof, and
wherein an unwrapped first phase response is obtained; subtracting
the determined second group delay from the phase response of the
transformed second head-related transfer function, whereby a
normalized second phase response results; unwrapping the normalized
second phase response, wherein phase jumps in the normalized second
phase response are eliminated by adding or subtracting a value of
360.degree. or multiples thereof, and wherein an unwrapped second
phase response is obtained; calculating an average linear delay
based on the determined first and second group delays; performing a
linear interpolation between the unwrapped first phase response and
the average linear delay according to the at least one processing
parameter, wherein a modified first phase response is obtained;
performing a linear interpolation between the unwrapped second
phase response and the average linear delay according to the at
least one processing parameter, wherein a modified second phase
response is obtained; assigning the modified first phase response
to the first filter with the first modified head-related transfer
function; and assigning the modified second phase response to the
second filter with the second modified head-related transfer
function.
5. The method according to claim 4, wherein the degree of binaural
virtualization is selectable by a single processing parameter, and
wherein in a first range of the processing parameter the
interpolating is performed between the amplitude response of the
transformed head-related transfer functions and the determined gain
factors, and wherein in a second range of the processing parameter
the interpolating is performed between the unwrapped phase
responses and the average linear delay.
6. The method according to claim 5, wherein the first range and the
second range do not overlap.
7. The method according to claim 1, wherein the degree of binaural
virtualization is selectable by at least two parameters that are
independent from each other.
8. The method according to claim 1, wherein the method is applied
to at least two different single channel input audio signals, and
wherein individual directions that may optionally differ from each
other and individual processing parameters for an individual degree
of binaural virtualization that may optionally differ from each
other are assigned to each of the at least two input audio
signals.
9. The method according to claim 8, wherein a first direction and
at least one first processing parameter for a first degree of
binaural virtualization are assigned to a first input audio signal,
and wherein a first and a second filter for the first input audio
signal are calculated, and wherein a second direction and at least
one second processing parameter for a second degree of binaural
virtualization are assigned to a second input audio signal, and
wherein a first and a second filter for the second input audio
signal are calculated, and wherein the first and second input audio
signals after filtering by their respective first filters are
superimposed to each other to obtain a first output signal for a
left-hand side, and wherein the first and second input audio
signals after filtering by their respective second filters are
superimposed to each other to obtain a second output signal for a
right-hand side.
10. The method according to claim 8, wherein the at least two
single channel input audio signals are received in a common
reception signal, the reception signal containing also information
about the directions and the processing parameters for a degree of
binaural virtualization.
11. The method according to claim 4, wherein an adjustable
additional delay is added to at least one of the modified first
phase response and the modified second phase response.
12. The method according to claim 1, wherein the determining the
first gain factor for the left side and the second gain factor for
the right side is performed according to a given or selectable
panning rule.
13. A non-transitory computer readable storage medium having stored
thereon instructions that when executed by a computer or processor
cause the computer or processor to perform the method according to
claim 1.
14. A device for processing an input audio signal to which at least
one processing parameter for a degree of binaural virtualization
and a direction are assigned, the device comprising: a database
adapted for providing a first head-related transfer function for a
left output signal for a left-side ear of a listener, and for
providing a second head-related transfer function for a right
output signal for a right-side ear of the listener, wherein the
head-related transfer functions correspond to the direction
assigned to the input audio signal; at least one gain factor
determining module adapted for determining a first gain factor for
the left side and a second gain factor for the right side, wherein
the first and second gain factors correspond to an amplitude
panning for the direction assigned to the input audio signal; at
least one first scaling and shifting module for the left side, the
first scaling and shifting module being adapted to bring an
amplitude response of the first head-related transfer function
closer to the first gain factor according to the processing
parameter by scaling and shifting, wherein an amplitude response of
a first modified head-related transfer function is obtained; at
least one second scaling and shifting module for the right side,
the second scaling and shifting module being adapted to bring an
amplitude response of the second head-related transfer function
closer to the second gain factor according to the processing
parameter by scaling and shifting, wherein an amplitude response of
a second modified head-related transfer function is obtained; where
at least in a first frequency range the amplitude responses for a
lower degree of binaural virtualization are brought closer to the
respective gain factor than for a higher degree of binaural
virtualization; a configurable first filter and a configurable
second filter adapted to filter the input audio signal; a first
filter configuration module adapted to calculate first filter
coefficients from the amplitude response of the first modified
head-related transfer function, and further adapted to configure
the first filter with the first filter coefficients; a second
filter configuration module adapted to calculate second filter
coefficients from the amplitude response of the second modified
head-related transfer function, and further adapted to configure
the second filter with the second filter coefficients; wherein said
filtering the input audio signal with the first and second
configurable filters results in an audio signal that is partially
binaurally virtualized according to the assigned degree.
15. The device according to claim 14, further comprising a
transformation module each for the left and the right side, the
transformation modules being adapted for transforming the first and
second head-related transfer functions into the frequency domain,
wherein transformed head-related transfer functions are obtained;
wherein the scaling and shifting modules scale and shift the
amplitude responses of the transformed head-related transfer
functions, wherein transformed amplitude responses of the modified
head-related transfer functions are obtained; and wherein the first
and second filter configuration modules calculate the filter
coefficients from the transformed amplitude responses.
16. The device according to claim 15, further comprising at least
one re-transformation module for performing inverse Fourier
transformation of said transformed amplitude responses of the
modified head-related transfer functions, wherein the filter
configuration modules calculate the filter coefficients from the
re-transformed amplitude responses.
17. The device according to claim 14, wherein said at least one
processing parameter for a degree of binaural virtualization and
said direction are assigned to the input audio signal within the
device, and wherein the device further comprises: an assignment
module adapted for performing said assigning the at least one
processing parameter for a degree of binaural virtualization and
the direction to the input audio signal.
Description
CROSS REFERENCE TO RELATED APPLICATION(S)
This application claims the benefit of the foreign priority of
German Patent Application No. 10 2019 135 690.3, filed on Dec. 23,
2019, the entirety of which is incorporated herein by
reference.
FIELD OF DISCLOSURE
The invention relates to audio signal processing for binaural
virtualization.
BACKGROUND
Various solutions are known for audio signals and their spatial
reproduction, which differ from each other fundamentally. Two
important principles are object-based audio, where the positions of
the audio sources are given, and channel-based audio, where the
positions of the loudspeakers or reproduction transducers
respectively are given. E.g. the well-known stereo and 5.1 surround
formats are channel-based. Here, a modification of the spatial
perception is commonly achieved by the so-called panning, whereby
the amplification or amplitude respectively of each reproduction
channel can be controlled. This method is therefore known as
amplitude panning. However, a considerably stronger spatial effect
can be achieved by binaural audio signal processing, generating
separate signals for the left and right ear. It uses head-related
transfer functions (HRTFs), which are also known as anatomical
transfer functions (ATFs).
FIG. 1 shows the principle of object-based binaural signal
processing. In order to binaurally reproduce the (mono) signal of
an audio source 11, it is filtered by a binaural filter 12a,12b
each for the left and right side. The binaural reproduction is done
through headphones 13 with two sound transducers. For binaurally
reproducing multiple audio sources 11.sub.1, . . . , 11.sub.N,
their signals are separately filtered
12a.sub.1,12b.sub.1,12a.sub.N,12b.sub.N and superposed for each
side, as shown in FIG. 2. The superposition may be done by
summation 14.sub.a, 14.sub.b. For a corresponding spatial
reproduction via loudspeakers, however, different filters are
required that have structures and features similar to binaural
filters. They are called transaural filters. FIG. 3 shows
transaural filters 12c, 12d filtering the (mono) signal of the
audio source 11 for spatial reproduction via loudspeakers 15a, 15b.
With binaural or transaural playback, the spatial effect is more
evident than with the usual stereo or 5.1 surround playback.
However, available audio signals often have stereo or 5.1 surround
format, and respective playback systems for these formats are
widespread. Due to the predefined fixed positions that loudspeakers
have in stereo or 5.1 surround systems respectively, each audio
channel can be assigned a direction from which the listener hears
the respective signal.
When using headphones, the respective signals of the channels can
be processed with a corresponding HRTF each for the left ear and
right ear in order to achieve the same hearing impression as with a
stereo playback via loudspeakers. In FIG. 2, the audio sources
11.sub.1, . . . , 11.sub.N may be the two channels of a stereo
signal, for example.
A particularly simple alternative for a spatial virtualization in
order to give the listener an impression of direction is panning.
With panning, the signals are not processed by HRTFs, but the
directional effect is only simulated by a sound level difference or
volume difference between the left ear and the right ear. Although
the spatial impression is less pronounced here, panning has the
advantage that each single sound source is perceived clearer. This
increases speech intelligibility, for example.
EP2258120 B1 shows the parallel use of equalization and binaural
filtering of surround audio signals for correcting the timbre. A
channel of a surround audio signal is, on the one hand, filtered by
a binaural filter for each side (left/right), and on the other hand
delayed and equalized by an equalizer for each side. The two
signals belonging to a respective same side are weighted and mixed,
wherein for one side an additional delay of the equalized signal is
inserted in order to generate interaural time differences (ITD).
Further, head-related transfer functions (HRTFs) may be modified in
order to compensate for timbral colorations. The head-related
transfer functions for the left and right sides are aligned with
each other such that the timbral coloration is reduced, which
however reduces also the spatial effect.
Binaurally reproduced signals are often perceived as unnatural or
unpleasant. Speech is sometimes difficult to understand and music
sounds strange and therefore uncomfortable, for example since
certain emphases intended by the musician are lost.
A further improvement of the spatial reproduction of audio signals
would be desirable.
SUMMARY OF THE INVENTION
At least this problem is solved by the present invention. Claim 1
discloses a method for processing an audio signal for binaural
virtualization, and in particular for partial binaural
virtualization, according to an embodiment of the invention. Claim
14 discloses a corresponding device, according to another
embodiment of the invention.
According to the invention, an improvement of the spatial
reproduction of audio signals may be achieved by filtering an audio
signal such that it is only partially binaurally virtualized. A
degree of binaural virtualization can be freely chosen for the
audio signal. In one embodiment, a control method is provided that
enables a smooth transition between a complete binaural
virtualization and a non-binaural virtualization that corresponds
to panning. This may be done during mixing, i.e. during the
authoring process, or later during post-processing or during
playback. Partially, the binaural virtualization may also be
effected by the temporal behavior of the filters for both sides,
i.e. their phase responses.
According to the invention, the signal processing includes
modifying the amplitude responses, corresponding to filtering
curves, and/or the phase responses of the HRTFs which correspond to
delays of the filters. The amplitude responses and phase responses
can in principle be modified independently from each other. Both
approaches can be used separately or together.
In particular, the signal processing for a transition from a
binaural to a non-binaural virtualization that is perceived as
smooth has at least two sections, in one embodiment. In a first
section beginning with a complete binaural virtualization and the
HRTFs that are usually used for that purpose, these HRTFs are
modified with a decreasing binaural virtualization, without
modifying their phase behavior or phase responses. In particular,
the "dynamic range" of each HRTF is successively reduced until it
is zero, i.e. until the HRTF value is frequency independent. This
frequency independent value is the gain factor that corresponds to
a stereo panning. The "dynamic range" of an HRTF is understood
herein as the difference between the highest and the lowest value
of the HRTF within a frequency range. In a second section, which in
one embodiment is adjacent to the first section, the phase behavior
of the HRTF, or the delay respectively, is modified. The delay may
be reduced, starting from a value that results from the "dynamic
reduced" HRTFs, down to zero (or another constant value that is
equal on both sides, left and right). At this point, the signal
processing corresponds to the known stereo panning.
Further advantageous embodiments are disclosed in the following
description and in the dependent claims.
An advantage of the invention is that audio objects or audio
channels can be virtualized to a greater or lesser extent, due to a
more binaural or more panning-like rendering or processing. In
other words, a degree of binaural processing of an audio object may
be freely chosen within a continuous range where the extremes are
e. g. a complete binaural processing and a classical amplitude
panning. This may be done by using e.g. a control device. A further
advantage is that different audio objects or audio channels may be
virtualized individually to different degrees and may then be
superposed to each other.
BRIEF DESCRIPTION OF THE DRAWINGS
Further details and advantageous embodiments are shown in the
drawings, wherein
FIG. 1 shows the known principle of object-based binaural signal
processing for a single audio source;
FIG. 2 shows the known principle of object-based binaural signal
processing for the superposition of multiple audio sources;
FIG. 3 shows the known principle of object-based transaural signal
processing;
FIG. 4 shows a flow-chart of a method according to an
embodiment;
FIG. 5 shows impulse responses and frequency responses of the
filters for different parameter values;
FIG. 6 shows a block diagram of a device according to an
embodiment;
FIG. 7 shows a flow-chart for determining the phase response of a
filter;
FIG. 8 shows a flow-chart according to an embodiment with an
interpolation of the phase response;
FIG. 9 shows, in an embodiment, a block diagram of a device for
superimposing multiple audio sources for playback via headphones,
wherein the audio sources are binaurally virtualized to different
degrees;
FIG. 10 shows, in an embodiment, a block diagram of a device for
superimposing multiple audio sources for playback via loudspeakers,
wherein the audio sources are binaurally virtualized to different
degrees; and
FIG. 11 shows a representation of different parameter ranges in an
embodiment where two processing parameters are used.
DETAILED DESCRIPTION
FIG. 4 shows, in an embodiment, a flow-chart of a method 400 for
processing a single channel input audio signal. A direction DIR and
a processing parameter P.sub.FC for a degree of binaural
virtualization are associated to the input audio signal, e.g.
during authoring. The input audio signal may be e.g. a single audio
object in an object-oriented audio format. However, it could also
be e.g. a channel (left/right) of a stereo signal. From the input
audio signal, output audio signals for playback at a left ear and a
right ear of a listener, respectively, are to be generated, e.g.
for headphones or for loudspeakers located near the ears. In a
first step 401, head-related transfer functions (HRTFs) for the
given target direction DIR are determined. These are a first
head-related transfer function HRTF.sub.L for a left side output
signal for the left ear of a listener and a second head-related
transfer function HRTF.sub.R for a right side output signal for the
right ear of the listener. The HRTFs may e.g. be coefficient data
sets retrieved from a database that has stored coefficients of a
plurality of HRTFs for different directions. If the coefficients of
the determined HRTFs are provided by the database in the time
domain format, they are in a second step 402 transformed into the
frequency domain by using a Fourier transform (FT). Otherwise, if
the data base provides already frequency domain coefficients, the
step 402 may be skipped.
As described above, a second, substantially simpler way of
processing is amplitude panning. A conventional amplitude panning
for the given target direction DIR is modelled 406, which includes
applying a first gain factor Gain_L for a left channel and a second
gain factor Gain_R for a right channel to the single channel input
audio signal. For example, for a certain given target direction DIR
the first gain factor Gain_L may be -10 dB and the second gain
factor Gain_R may be -6 dB, leading to a simple spatial
virtualization of the audio object at a position rather to the
right. For a target direction DIR that is just in front of the
listener or behind the listener, both gain factors are usually
essentially equal.
In the next step, the amplitude responses of the transformed
head-related transfer functions are adjusted 403, 408 to the
respective gain factors according to the processing parameter
P.sub.FC for a degree of binaural virtualization. That is, the
amplitude response of the first head-related transfer function
HRTF.sub.L is brought closer to the first gain factor Gain_L to an
extent depending on the processing parameter P.sub.FC, and the
amplitude response of the second head-related transfer function
HRTF.sub.R is brought closer to the second gain factor Gain_R to an
extent depending on the processing parameter P.sub.FC. As explained
below in more detail, this can be understood as scaling or
compressing the amplitude responses of the HRTFs, approaching them
to respective frequency independent target values and resulting in
a first modified head-related transfer function HRTF.sub.L,mod1 and
a second modified head-related transfer function HRTF.sub.R,mod1.
This adjustment or approaching 403, 408 is stronger if the intended
degree of binaural virtualization is lower, and vice versa. In an
embodiment, the modified head-related transfer functions for a
minimum degree of binaural virtualization are identical with the
gain factors Gain_L, Gain_R, while for a maximum degree of binaural
virtualization they are identical with the original head-related
transfer functions. In an embodiment, the amplitude responses of
the original head-related transfer functions are first, in a step
403, scaled or reduced according to the processing parameter
P.sub.FC and then, in a further step 408, the scaled or reduced
head-related transfer functions are adjusted or approached to the
gain factors Gain_L, Gain_R by shifting (ie., by amplifying or
attenuating the signals). In other embodiments, these steps 403,
408 may be swapped or may be executed simultaneously, or otherwise
embedded in any processing.
Finally, filtering functions for the first and second modified
head-related transfer functions HRTF.sub.L,mod1, HRTF.sub.R,mod1
are calculated 411 and transformed back to the complex spectrum.
Ten, the filtering coefficients for implementing the filters are
calculated 413. A first filter is implemented according to the
first modified head-related transfer function HRTF.sub.L,mod1 and a
second filter is implemented according to the second modified
head-related transfer function HRTF.sub.R,mod1. Optionally, the
modified head-related transfer functions HRTF.sub.L,mod1,
HRTF.sub.R,mod1 may be transformed 412 into the time domain by an
inverse Fourier transform before.
In an embodiment, the phase response of the first or second filter,
respectively, results directly from the respective first or second
modified head-related transfer function HRTF.sub.L,mod1,
HRTF.sub.R,mod1. In another embodiment, however, the phase response
of the first or second filter, respectively, may be modified. This
modification may be based on the above-mentioned processing
parameter P.sub.FC but it may also be based on a different second
processing parameter P.sub.TC. Further details are explained
below.
FIG. 5 shows, in one embodiment, impulse responses and frequency
responses of exemplary filters for different parameter values. In
this example, a processing parameter P.sub.C for a degree of
binaural virtualization is composed from the above-mentioned first
processing parameter P.sub.FC and a second processing parameter
P.sub.TC. The first processing parameter P.sub.FC modifies the
filters' amplitude response or frequency response and may be
referred to as "frequency clarity". The second processing parameter
P.sub.TC modifies the filters' phase response and may be referred
to as "time clarity". Table 1 shows exemplarily a relationship
between the total processing parameter P.sub.C, the first
processing parameter P.sub.FC and the second processing parameter
P.sub.TC.
TABLE-US-00001 TABLE 1 Adjacent parameter sections Range of values
for P.sub.C B.sub.1 B.sub.2 (Thr < 100%) (0 .ltoreq. P.sub.C
.ltoreq. Thr) (Thr .ltoreq. P.sub.C .ltoreq. 100%) P.sub.FC 0% . .
. 100% 100% P.sub.TC 0% 0% . . . 100%
This relationship is depicted in FIG. 11, where the value range of
the processing parameter P.sub.C comprises two ranges or sections.
A first range or section B.sub.1 starts from P.sub.C=0 (or 0%) and
ranges up to a threshold Thr. A second range or section B.sub.2
ranges from the threshold Thr up to P.sub.C=1 (or 100%). The
threshold may be, e.g., Thr=0.7 or Thr=0.6, . . . , 0.8, or
similar. In the first section B.sub.1, which is wider than the
second section B.sub.2 in this example, only the first processing
parameter P.sub.FC is modified. In the second section B.sub.2, only
the second processing parameter P.sub.TC is modified. In the first
section B.sub.1, the binaural virtualization and thus the
spatialization effect is stronger, while in the second section
B.sub.2 it is weaker. Overall, a change in the spatial effect that
is perceived as uniform or smooth results over the control range of
the processing parameter P.sub.C. In this particular example, the
change in the spatial effect is a decreasing spatial impression
with an increase of the processing parameter P.sub.C. However, it
is clear that other implementations are possible where the spatial
effect increases with an increase of the parameter.
This relationship is depicted in FIG. 5, where impulse responses
and frequency responses (i.e. amplitude responses) of the filters
for the first and second modified head-related transfer functions
HRTF.sub.L,mod1, HRTF.sub.R,mod1 are exemplarily shown for
different values of the processing parameter P.sub.C. FIG. 5a)
shows the situation for P.sub.C=0.0, i.e. a maximum degree of
binaural virtualization. This corresponds to P.sub.TC=P.sub.FC=0.0,
and the amplitude responses shown in the lower part fully
correspond to the amplitude responses of the original head-related
transfer functions HRTF.sub.L, HRTF.sub.R, both for the side facing
the sound source ("ipsilateral") 51i and for the side facing away
from the sound source ("contralateral") 51c. In the time domain,
these frequency responses correspond to the impulse responses for
the ipsilateral side 51i.sub.t and for the contralateral side
51c.sub.t that are shown in the upper part of FIG. 5a). The level
difference (interaural level difference, ILD) and the runtime
difference (interaural time difference, ITD) between the first two
peak values 51i.sub.t, 51c.sub.t are clearly visible. This
corresponds to a sound signal being weaker and arriving later at
the contralateral ear than at the ipsilateral ear. Also an initial
delay of about 80 ms prior to the first peak value 51i.sub.t is
clearly visible, while the runtime difference is about 10-15
ms.
FIG. 5b) shows the responses for P.sub.C=0.2. The processing
parameter P.sub.C is in the first section B.sub.1. The resulting
effect is easier visible in the lower diagram showing the frequency
response, namely in that the magnitude of the frequency response is
scaled or reduced, respectively. That is, the difference between
minimum and maximum values is smaller than in FIG. 5a) both for the
ipsilateral 52i and the contralateral side 52c. At the same time,
the curves of the diagram are shifted towards lower values (as
compared to the original curves 51i, 51c), which is visible
particularly for the lower frequencies. However, this shift applies
to the complete respective curve 52i,52c (at least the audible
spectrum portion). This effect is not so clearly visible in the
time domain, as the upper part of FIG. 5b) shows.
Also in FIG. 5 c) for P.sub.C=0.4, the processing parameter P.sub.C
is in the first section B.sub.1. The effect described above for
FIG. 5b) is more pronounced, i.e. the head-related transfer
functions 53i,53c for the ipsilateral side and the contralateral
side are more reduced and more shifted. Together with the frequency
response, also the phase response changes. Due to the modified
frequency and phase responses, effects are now visible also in the
time domain, namely an increase of signal portions occurring before
the first peak value 53i.sub.t. In FIG. 5d) for P.sub.C=0.6, these
changes continue to become more evident in that the frequency
responses 54i,54c already show a magnitude that is clearly reduced
or scaled, respectively. In the time domain however, the delay
between the respective first two peak values is substantially
unchanged for different values of P.sub.C=0.0, . . . , 0.6
corresponding to FIG. 5a)-d).
FIG. 5e) shows the situation for P.sub.C=0.8. The processing
parameter P.sub.C is here at the edge of the first section B.sub.1
or already in the second section B.sub.2. As shown in the frequency
response in the lower diagram, the curves are flat, i.e. the
head-related transfer functions 55i,55c for the ipsilateral side
and the contralateral side at least in the frequency range up to 10
kHz have assumed frequency independent values that correspond to
gain values of a stereo amplitude panning. The curves from FIG.
5a)-d) have gradually approached these values. Between P.sub.C=0.6
and P.sub.C=0.8, the second section B.sub.2 begins. Although the
phase responses are not depicted directly, it is visible in the
time domain diagram shown in the upper part of FIG. 5e) for
P.sub.C=0.8 and FIG. 5f) for P.sub.C=1.0 that the impulse responses
of the two sides approach each other (i.e. the time between the
first and second peak values 55i.sub.t,55c.sub.t is reduced) until
finally both peaks are equal for P.sub.C=1.0. This is the main
effect in the second section B.sub.2, while the frequency responses
55i,56i and 55c,56c remain substantially unchanged, namely in that
they represent constant gain factors. At this point, which is shown
in FIG. 5f), the processing parameter P.sub.C has the value 1.0
(100%) and the audio signal processing fully corresponds to stereo
amplitude panning, while in FIG. 5a) for a processing parameter
value of P.sub.C=0.0 (0%) the audio signal processing fully
corresponds to binaural processing.
As mentioned above, the processing parameter P.sub.C for a degree
of binaural virtualization in this example is composed of two
separate sections B.sub.1,B.sub.2, which may be expressed by two
separate processing parameters P.sub.FC, P.sub.TC. This embodiment
is particularly advantageous since it results in a change of the
spatial effect that is perceived as even. Alternatively, also other
variants are possible, e.g. the following for
Thr.sub.2<Thr.sub.1:
TABLE-US-00002 TABLE 2 Overlapping parameter sections Value range
P.sub.C (for Thr.sub.1, Thr.sub.2 < 100, Thr.sub.2 <
Thr.sub.1) 0 .ltoreq. P.sub.C .ltoreq. Thr.sub.1% Thr.sub.2
.ltoreq. P.sub.C .ltoreq. 100% P.sub.FC 0% . . . 100% 100% P.sub.TC
0% 0% . . . 100%
Here, the sections of the first processing parameter P.sub.FC and
second processing parameter P.sub.TC overlap and there is a middle
range between Thr.sub.2 and Thr.sub.1 in which both parameters are
modified. In some cases. e.g. based upon individual preference,
also this variant may be perceived as advantageous. In any case,
the respective processing parameter P.sub.C, P.sub.TC, P.sub.FC may
in principle be adjusted continuously from 0% to 100%.
FIG. 6 shows a block diagram of a device 600 for processing a
single-channel input audio signal 11, according to an embodiment.
At least one processing parameter P.sub.C, P.sub.TC, P.sub.CF for a
degree of binaural virtualization and a direction DIR is associated
to the input audio signal 11. The device 600 comprises a storage or
database 601 for storing and providing head-related transfer
functions, including those head-related transfer functions that
correspond to the direction DIR that is associated to the input
audio signal 11. These are a first head-related transfer function
HRTF.sub.L,ori for a left side output signal for a left ear of a
listener and a second head-related transfer function HRTF.sub.R,ori
for a right side output signal for a right ear of the listener.
Further, the device 600 comprises at least one gain factor
determining module 606L,606R for determining a first gain factor
Gain_L for the left side and a second gain factor Gain_R for the
right side, which gain factors correspond to an amplitude panning
for the direction DIR that is associated to the input audio signal
11. A rule or an algorithm for the amplitude panning may be
predefined or selectable, such as e.g.
Gain_L=0.5*(1+sin(.quadrature..sub.azimuth,L)) and
Gain_R=0.5*(1-sin(.quadrature..sub.azimuth,R)), wherein
.quadrature..sub.azimuth .di-elect cons.[-180.degree., . . . ,
180.degree. ] is the respective angle to the front direction. In
other embodiments, other audio virtualization rules and in
particular other panning rules may be used, which may be based for
example on A-B miking (time-of-arrival stereophony) with a given
distance between the microphones (base distance). For a pure
amplitude panning, the gains are to be set to Gain_L=Gain_R=0.
Further, the device 600 comprises a transformation module 603L,603R
each for Fourier transforming 730 the first and second head-related
transfer functions HRTF.sub.L,ori, HRTF.sub.R,ori into the
frequency range, resulting in respective transformed transfer
functions HRTF'.sub.L,ori, HRTF'.sub.R,ori. Then the amplitude
responses and the phase responses of the transformed transfer
functions HRTF'.sub.L,ori, HRTF'.sub.R,ori may be processed in
principle independent from each other.
In an embodiment, the device 600 comprises two scaling and shifting
modules 604L, 604R, 608L, 608R, one for each side, left and right.
A first scaling and shifting module 604L, 608L for the left-hand
side adjusts the amplitude response of the first head-related
transfer function HRTF'.sub.L,ori to be closer to the first gain
factor Gain_L according to a processing parameter P.sub.FC by
scaling and shifting, for instance according to
Mag_out_L=(1-P.sub.FC)*mag.sub.4L+P.sub.FC*Gain_L. This results in
an amplitude response Mag_out_L of a first modified head-related
transfer function HRTF.sub.L,mod1. Likewise, a second scaling and
shifting module 604R, 608R for the right-hand side adjusts the
amplitude response of the second head-related transfer function
HRTF'.sub.R,ori to be closer to the second gain factor Gain_R
according to the processing parameter P.sub.FC by scaling and
shifting, for instance according to
Mag_out_R=(1-P.sub.FC)*mag.sub.4R+P.sub.FC*Gain_R. This results in
an amplitude response Mag_out_R of a second modified head-related
transfer function HRTF.sub.R,mod1. As described above, the binaural
virtualization effect is the stronger, the closer the amplitude
responses Mag_out_L, Mag_out_R of the modified head-related
transfer functions HRTF.sub.L,mod1, HRTF.sub.R,mod1 are to the
original head-related transfer functions HRTF.sub.L,ori,
HRTF.sub.R,ori. In other words, the approaching of the amplitude
responses to the gain factors Gain_L, Gain_R is stronger pronounced
for a lower degree of binaural virtualization than for a higher
degree of binaural virtualization. This applies at least in a
limited frequency range, e.g. below a certain maximum frequency
(Nyquist frequency); it needs not necessarily be valid over the
full frequency range. Therefore it may be sufficient to apply the
processing in the limited frequency range.
The device further comprises for each side a configurable filter
613L, 613R for filtering the input audio signal 11 to obtain the
left output signal and right output signal, and a filter
configuration module 611L, 611R for each of the configurable
filters. The first filter configuration module 611L calculates
first filter coefficients from the amplitude response Mag_out_L of
the first modified head-related transfer function HRTF.sub.L,mod1,
and the first configurable filter 613L is configured with the first
filter coefficients. The second filter configuration module 611R
calculates second filter coefficients from the amplitude response
Mag_out_R of the second modified head-related transfer function
HRTF.sub.R,mod1, and the second configurable filter 613R is
configured with the second filter coefficients. By filtering the
input audio signal 11 with the first and the second configured
filters 613L, 613R, audio signals 11.sub.out,L,11.sub.out,R are
created that are partially binaurally virtualized to a certain
degree, according to the associated parameter. They may be
reproduced, e.g. via headphones. Each of the above-mentioned
modules and filters individually or together may be implemented
e.g. by one or more software-configurable processors or
computers.
In the embodiment as described above, mainly the amplitude
responses of the head-related transfer functions may be modified.
In another embodiment, the phase responses or delays respectively
of the head-related transfer functions may be modified. Both
embodiments are independent from each other and may be combined.
Therefore both are shown together in FIG. 6. The following refers
also to FIG. 7 showing a flow-chart of a method 700 for determining
the phase response of a configurable filter 613L, 613R. The first
steps for determining 710 the head-1o related transfer function for
the given target direction DIR and performing a Fourier
transformation 730 have already been mentioned above.
For modifying the phase responses or delays respectively of the
head-related transfer functions HRTF.sub.L,ori, HRTFa.sub.R,ori,
the device 600 may optionally comprise a delay determining module
602L, 602R each for calculating 720 the respective linear delay or
group delay LPD.sub.2L, LPD.sub.2R of the head-related transfer
functions HRTF.sub.L,ori, HRTF.sub.R,ori for the left and right
sides as received from the database. Alternatively, these values
may also be received from the database, so that they need not be
re-calculated again with each call. The Fourier transformation 730
may be performed before or after or concurrently with the step 720
of determining the linear delays. The device 600 further comprises
an MLV calculation module 609 for calculating a mean or average
linear delay MLV from the linear delays LPD.sub.2L, LPD.sub.2R of
the two sides, for example according to
MLV=0.5*(LPD.sub.2L+LPD.sub.2R).
Further, the device 600 comprises a subtraction module 605L, 605R
each for subtracting 740 the respective group delay LPD.sub.2L,
LPD.sub.2R from the phase response of the transformed head-related
transfer function HRTF'.sub.L,ori, HRTF'.sub.R,ori, whereby a
normalized first phase response and a normalized second phase
response are generated. Since these normalized phase responses may
contain phase jumps of 360.degree., they are unwrapped 750. That
is, such phase jumps are eliminated from the phase responses by
adding or subtracting 360.degree. or multiples thereof. Unwrapping
may also include changing absolute jumps greater than 180.degree.
to their 360.degree. complement. The resulting so-called unwrapped
phase responses Ang_L, Ang_R are free from phase jumps. The
unwrapped phase responses Ang_L, Ang_R are then scaled 760 by
interpolation through phase interpolation modules 610L, 610R. The
interpolation may be a linear interpolation between the respective
unwrapped phase response Ang_L, Ang_R and the average linear delay
MLV according to the processing parameter P.sub.C, P.sub.TC for a
certain degree of binaural virtualization, e.g. for the left-hand
side according to LinearDelayL=(1-p.sub.TC)*LPD.sub.2L+p.sub.TC*MLV
Ang_out_L=(1-p.sub.TC)*Unwrap(ang5L-LPD.sub.2L)+p.sub.TC*(LP.sub.L+Linear-
DelayL) where ang5L is the phase response of the head-related
transfer function HRTF'.sub.L,ori after Fourier transformation and
before unwrapping, and LPL is an optional additional delay. This
results in the modified phase responses Ang_out_L, Ang_out_R that
are then fed to the filters 613L, 613R. The phase responses may
optionally be modified by adding 770 a (possibly constant) delay
LP.sub.L, LP.sub.R, which may be received from a panning module
607L, 607R that models a runtime panning. The respective additional
delay for the left and right side may depend on the direction
DIR.
From the modified phase responses Ang_out_L, Ang_out_R and/or the
interpolated amplitude responses Mag_out_L, Mag_out_R, the modified
head-related transfer functions HRTF.sub.L,mod1, HRTF.sub.R,mod1 or
their coefficients respectively for configuring the filters 613L,
613R may be generated in the filter configuration modules 611L,
611R. Before configuring the filters, the modified filtering
functions including the modified phase responses Ang_out_L,
Ang_out_R may optionally be re-transformed 780 into the time domain
by inverse Fourier transformation 612L, 612R if required.
FIG. 8 shows a flow-chart of a method 800 including an
interpolation of the phase response, according to an embodiment.
Compared with the flow-chart in FIG. 4, additional steps are
comprised for normalizing and unwrapping 405 the phase responses of
the head-related transfer functions, as described above,
determining 404 the average linear delay (or group delay
respectively) MLV and adding it 409 to the phase responses. Then
follows an interpolation 410 according to the processing parameter
P.sub.TC, as described above, either towards the average linear
delay MLV or, optionally, towards a different runtime panning that
may be modelled separately 407. The respective modelled runtime
values may be retrievable from a memory.
From the interpolation results the desired phase response
Ang_out_L, Ang_out_R, which is combined with the desired amplitude
response Mag_out_L, Mag_out_R so as to obtain the target
head-related transfer functions HRTF.sub.L,mod1, HRTF.sub.R,mod1.
Thus, the filtering function is formed or determined respectively
411, from which then the filtering coefficients are determined 413
directly or after an optional inverse Fourier transformation 412,
612.
FIG. 9 shows, in an embodiment, a block diagram of a device for
superimposing multiple audio sources that may be differently
binaurally virtualized for playback via headphones. Multiple input
audio signals 11.sub.1,11.sub.2, . . . , 11.sub.N from the audio
sources may be received in one or more reception signals. To each
input audio signal 11.sub.1,11.sub.2, . . . , 11.sub.N may be
assigned not only an individual direction DIR.sub.1, DIR.sub.2, . .
. , DIR.sub.N, but also an individual degree of virtualization by
means of one or more individual processing parameters P.sub.FC,1,
P.sub.FC,2, . . . , P.sub.FC,N, P.sub.TC,1, P.sub.TC,2, . . . ,
P.sub.TC,N, as described above. The direction and, in principle,
the processing parameters may vary over time (e.g. depending on a
video scene). The respective filtered audio signals for each side
are superimposed to each other 14.sub.a,14.sub.b and fed to the two
sides of a headphone 13. Thus, it is possible to virtualize certain
audio objects different from other audio objects, for example for
the soundtrack of a movie. For example, speech intelligibility may
be improved by assigning a lower degree of binaural virtualization
to speech than to music or ambient sound. Correspondingly, it is
also possible to classify input audio signals e.g. by assigning
them classification parameters P.sub.Typ such that the same
processing parameters P.sub.C, P.sub.TC, P.sub.FC apply to all
audio objects of a given class and different classes of audio
signals have different processing parameters. This enables an
automatic gradual binaural virtualization of audio signals (e.g.,
all speech signals are weakly binaurally virtualized while all
ambient sounds and/or music are strongly binaurally virtualized). A
classification may also be performed automatically, based on the
audio signal. E.g. artificial intelligence may be used for
differentiating between music, speech, ambient noises, effects
and/or other audio classes. The corresponding parameters may then
be assigned automatically to the audio signals, depending on the
classification.
The device for superimposing multiple audio sources may comprise a
plurality of separate devices 600 for processing single channel
input audio signals each, as described above. The devices may also
be integrated into a single device, however, which may lead to
synergy effects (e.g. a shared database). Further, there may be
cases where it is useful to perform the above-described processing
for only one of the sides, left or right, while the audio signal
for the other side may be processed differently.
It should be noted that the invention is not only applicable for
gradual binaural virtualization, but also for gradual transaural
virtualization. A device 600 for binaural virtualization differs
from a device for transaural virtualization mainly in the type of
transfer functions that are provided by the database. FIG. 10
shows, in an embodiment, a block diagram of a device 900 for
superimposing multiple audio sources, which are binaurally (or
rather transaurally) virtualized to different degrees, for audio
playback via loudspeakers 15a, 15b. In principle, it corresponds in
structure and function to the example shown in FIG. 9, except that
the transfer functions or filtering functions and the output
transducers are different.
The processing parameters P.sub.C, P.sub.TC, P.sub.FC or
classification parameters P.sub.Typ respectively may be stored as
metadata for later use in the input audio signals, e.g. for
real-time rendering in a playback device during reproduction. Thus,
for example, a system may be realized in which a head tracker
provides additional information about the position and orientation
of the listener. Apart from the real-time processing, the used
parameters may also be defined and stored in advance, e.g. by a
sound engineer. Tus, the invention may provide to sound engineers
new tools for continuously controlling a gradual degree of tonal
changes with respect to spectrum and/or phase. Moreover, the
parameter values and their changes over time may be stored. Instead
of assigning only a single value to the whole audio signal, the
signal may be subdivided into blocks (e.g. of 1 ms length or for
the length of a scene) and individual parameter values may be
assigned to each of these blocks. Audible artifacts may be
minimized by suitable windowing and cross-fading.
The invention is particularly advantageous for audio processing
devices, for example. It may be implemented based on a configurable
computer or processor, in an exemplary embodiment. The
configuration may be achieved by a computer-readable storage medium
having stored thereon instructions that when executed on a computer
cause the computer to perform a method as described above.
Various combinations of the above-described features with each
other or with further features are considered to be within the
scope of the invention, even if such combination is not expressly
mentioned herein.
* * * * *